Resistor unit and method of making same



F. M. MINKS RESISTOR UNIT AND METHOD OF MAKING SAME April 22, 1969 Filed April 1, 1966 INVENTOR /20Y0 M. Alma 4/far e s United States Patent 3,440,589 RESTSTDR UNIT AND METHOD OF MAKING SAME US. Cl. 333-22 13 Claims ABSTRACT OF THE DISCLOSURE This disclosure is directed to a nickel wire wound on and covered with ceramic to form a resistor embedded in a controlled thermal impedance mass. The impedance mass includes hollow alumina bubbles which are mixed with a silicone rubber binder to maintain good thermal contact and controlled heat transfer. The resistor is enclosed within an outer tubular shell, one end of which is rolled over. A plastic member having an inner conical surface is inserted in the shell and held therein by the rolled end. The shell is filled with the mixture of the alumina bubbles and the silicone rubber. The resistor is then inserted into the tube with the conical inner surface of the plastic end directing the resistor lead through the assembly. The opposite end is then closed with a plastic member.

This invention relates to a current responsive resistor unit including a thermal impedance to provide a varying resistance with current and particularly to such a resistor which can be employed in severe vibration and temperature environments.

The use of resistors having high temperature coeflicients to control the current in a circuit in accordance with temperature and current levels is known. Although various resistors are available, they are not readily adapted as a practical matter to severe environmental conditions particularly where they are subjected to vibration and the like. For example, applicants copending application entitled, Triggered Ignition System, filed on Oct. 4, 1965, with Ser. No. 492,571, now abandoned, and assigned to the same assignee as the present application discloses a highly practical capacitor discharge ignition system wherein a transistor is connected as a part of a triggered blocking oscillator. The transistor is supplied with a turn-on current from the battery or other suitable DC. power supply. With low battery voltage and low engine speed such as encountered under engine cranking conditions, a relatively low resistance is desirably inserted in the base circuit to provide the desired turn-on current. However, as the engine speed increases and/ or the battery voltage goes up, the resistance is preferably increased to limit the current to approximately that required to drive the transistor. This is of course desirable to prevent excessive power dissipation in the base circuit and adverse overloading of the components.

However, a suitable resistor having the desired sharp change in resistance characteristics with current was not available, particularly in the potted construction of the above entitled application wherein a potted ignition system is mounted as a part of an outboard motor and therefore subjected to great amounts of vibration. The available units generally employ air as the thermal impedance. In p nom [Ho s/ s ponod a u; stamp 12 qons poquro 0; 19pm require scaling to prevent introduction of potting material and further mounting of the resistor therein to prevent vibration damage.

The present invention is thus directed to a resistor unit or element having a high temperature coefiicient resistor embedded in a controlled thermal impedance mass which is particularly adapted and constructed to form a part of a potted ignition system and the like.

Generally, in accordance with the present invention, the resistor is embedded in a mass of hollow members, preferably ceramic bubbles of alumina or the like. The bubbles are mixed with a suitable binder, preferably silicone rubber or the like, in a suflicient amount to maintain or hold the bubbles together; for example, similar to a mass of wet sand. The mass is not saturated with the binder however in order to establish a number of air spaces within the mass which prevent expansion and contraction of the silicone rubber with temperature from producing destructive pressures. The spongy characteristic of the mass maintains good thermal contact and controlled heat transfer thus insuring that the temperature of the resistance element varies in the desired manner with variation in current through the element.

In a preferred construction, a nickel wire wound on and covered with ceramic provides a resistor which must reach a temperature in the order of 350 C. without degradation of the insulation to provide the desired action. The present invention has been found to operate highly satisfactorily at such temperatures over extended and practically indefinite periods of time.

In a preferred construction, the resistor is enclosed within an outer tubular housing or shell one end of which is rolled over. A plastic member having an inner conical surface is inserted in the shell and held therein by the rolled end. The shell is filled with the mixture of the ceramic bubbles and the binder. The resistor is then inserted into the tube with the conical inner surface of the plastic end directing the resistor lead through the assembly. Alternatively, the resistor can be inserted in the shell and the mass then inserted. The opposite end is then closed with a suitable plastic member and the total mass allowed to set to firmly interconnect the elements to the casing and to each other.

The drawing furnished herewith illustrates preferred constuctions of the present invention in which the above advantages and features are clearly disclosed as well as others which will be clear from the following description of the drawing.

In the drawing:

FIG. 1 is a cross sectional view of a resistor unit constructed in accordance with the present invention;

FIG. 2 is an exploded view of the elements shown in FIG. 1 to more clearly disclose the components of the preferred construction; and

FIG. 3 is a substantially enlarged view of the thermal insulation shown in FIGS. 1 and 2.

Referring to the drawing and particularly to FIG. 1, a small elongated resistor 1 is illustrated having end leads 2 and 3 projecting axially from the opposite ends thereof. The resistor 1 is concentrically disposed within a tubular shell 4 of aluminum or the like with the end leads 2 and 3 projecting outwardly through the opposite ends of the shell 4. The ends of the shell are closed by small plastic end caps 5 and 6 to seal the ends and hold a thermal impendance material 7 within the shell 4 completely encircling and enclosing the resistor 1.

The illustrated resistor 1 is a commercially available unit, generally including a nickel wire 8 spirally wound upon a central ceramic core 9 and connected in any suitable manner to leads 2 and 3. An outer ceramic protective coating covers the nickel wire 8 and the core 9. Leads 2 and 3 project generally axially outwardly of the ends of the core 1.

The shell 4 is formed of any suitable metal such as aluminum or the like and has the one end rolled over slightly as at 11 to provide a support for the inner plastic end cap 5, as presently described.

The plastic end caps 5 and 6 are shown as identical members having an outer cone-shaped nose 12 integrally formed with a cylindrical end 13. The diameter of the cylindrical end .13 generally corresponds to the inner diameter of the shell 4 and the end caps 5 and 6 are assembled with the cylindrical ends 13 within the corresponding outermost ends of the shell 4 and with the coneshaped nose 12 projecting outwardly therefrom. The end caps 5 and 6 further include internal cone-shaped surfaces 14 to form a guide for the end lead 2 during assembly, as hereinafter described.

As most clearly shown in FIG. 3, the thermal impedance material 7 which completely fills the shell 4 to enclose resistorl is formed of a mass of fine hollow balls 15 of alumina or other similar high temperature material. The hollow balls 15 are held together by a suitable high temperature binder 15 such as silicone rubber.

In accordance with an important feature of the present invention, the binder 16 only partially fills the space between the hollow balls 15 to provide a plurality of air spaces 17 within the insulation. This permits the binder to expand and contract with temperature changes while maintaining good thermal contact with the surfaces of the several other components. The hollow balls of alumina do not essentially change configuration or size with temperature.

In assembly, the :balls 15 and the liquid silicone rubber 16 are intermixed as a wet soggy mass similar to damp soggy sand. Generally, the mixture may be such that the surfaces of the balls are coated with the binder while the space therebetween is free. The end cap 5 is assembled with the shell 4 with the rolled end 11 supporting the cap within the shell. The mixture of the alumina balls 15 and the silicone rubber v16 is inserted into the shell 4 to substantially fill the shell. The resistor 1 is inserted into the wet mass within the shell 4 with the lead 2 moving downwardly through the shell 4 and being directed through the exit opening of cap 5 by the inner coneshaped surface 14. The cap 6 is then slipped over the end of the opposite lead 3 with the cylindrical portion forced into the corresponding cylindrical end of the shell 4. The silicone rubber 16 is then cured as by heating or the like. When cured, the silicone rubber binds to the shell 4 and to the end caps to firmly interconnect the several members and establish stable thermal impedance between the resistor 1 and the shell.

It has been found that the embedding of the resistor 1 within the thermal material 7 consisting of the small ceramic balls 15 and the limited amount of silicone binder 16 maintains highly desirable operating characteristics. In particular, the resistance unit can be operated at a high temperature level without degradation of the material and in a potted assembly such as diagrammatically shown in FIG. 1. The environmental surroundings and particularly vibration of the unit will not adversely affect its operation and consequently provides a highly practical resistance element as a part of an ignition system for outboard motors and the like.

In FIG. 1, the unit is shown disposed within a recess within a metallic housing or base 18. The unit is secured therein by a suitable adhesive 19 such as an epoxy to firmly attach the unit in physical and heat exchange relation to the base 18. A suitable potting compound 20 completely covers the unit as well as any associated components, such as a transistor 21, to form a potted assembly, such as more fully described and shown in applicants previously referred to applicuti on.

The particular attachment of the shell 4 to the base 18, which serves as a heat sink, will substantially eliminate transfer of the temperature variation of the unit to the other components such as transistor 21 and thereby further adapt the unit for incorporation in an ignition system and the like.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A resistor unit, comprising a resistance element with a positive temperature coefficient of resistance, and

thermal impedance material surrounding said element,

said thermal impedance material comprising a mass of hollow members formed of a high temperature material and partially bound together by a high temperature binder to define a mass having substantial air spaces within the mass.

2. The resistor unit of claim 1 wherein said members are generally spherical ceramic members.

3. The resistor unit of claim 1 wherein said binder is a silicone rubber.

4. The resistor unit of claim 3 wherein the hollow members are formed of alumina.

5. The resistor unit of claim 1 wherein an outer metallic tube is provided having the resistance element centrally located therein, said thermal impedance material is within the tube and about said element.

6. The resistor unit of claim 1 wherein said hollow members are alumina and said binder is silicone rubber and having a metallic outer housing, a heat sink, and securement means to secure the housing in heat exchange relation to the heat sink.

7. The resistor unit of claim 6 wherein the heat sink is a metallic mounting member and the securement means is an adhesive.

8. The resistor unit of claim 6 having a potting compound enclosing the resistor unit.

9. The resistor unit of claim 8 having a temperature sensitive solid state component in the potting compound.

10. The resistor unit of claim 1 for use in vibrating environments, wherein said resistance element has a high temperature coefficient of resistance, and includes an outer metallic tube open at the opposite ends, end caps located within the opposite ends of the tube, said thermal impedance filling said tube and having said binder adhered to the tube and end caps to form a unitary unit.

11. The resistor unit of claim 1, including a metallic tubular shell having the opposite ends closed by plastic end caps with the leads projecting through the end caps, and said thermal impedance fills the shell and binds to the end caps and the shell.

12. The resistor unit of claim 11 wherein the hollow members are spherical ceramic members and the binder is a silicone rubber.

13. The method of forming a resistor unit having a resistance element with oppositely extending leads embedded in a thermal mass held within a tubular shell closed at the opposite ends by cap members, comprising the steps of mixing hollow members with a settable binder to form the thermal mass,

assembling one end cap with the tubular shell,

filling the tube with the thermal mass,

inserting the resistance element through the open end of the shell,

assembling the second end cap with the tubular shell,

and

setting said binder to interconnect the hollow members to each other and to the resistance element, the end caps and the shell.

(References on following page) References Cited UNITED STATES PATENTS Lien 29-6 14 6 FOREIGN PATENTS 475,667 11/1937 Great Britain.

REUBEN EPSTEIN, Primary Examiner.

5 US. 01. X.R. 

